Atomic memory using higher-order nonlinear processes

Abstract

Quantum memories are essential for many tasks in quantum information, especially for long-distance quantum communication protocols, where optical memory would be essential to the synchronization of the different segments of a linear chain of pairs of entangled atomic en- sembles. Implementations of atomic-ensemble quantum memory have been carried out using different techniques. Many of them employ essentially first-order ground-state coherences to store the phase information of the incident fields, being the overall nonlinear process of stor- age and retrieval characterized by the third-order nonlinear susceptibility χ(³) of the atomic medium, which is connected to four-wave mixing. The present work explores new possibilities for the selective storage of higher order nonlinearity in a sample of cold atoms confined in a magneto-optical trap. Particularly, we demonstrate the storage of a third-, fifth-, and seventh- order nonlinear atom-light interaction into the Zeeman structure of hyperfine cesium 6S½, F = 3 ground state. The writing process creates gratings associated with these higher-order nonlinear processes, which can be selectively read by the read beam after a controlled storage time. During this writing process, two different types of grating are formed: coherence and population gratings. Each of them stores only specific orders of nonlinearities, which implies that part of the information is stored in the populations and the other part in the ground-state coherence, leading to particular features in the signal of each order. We also demonstrated that such memories were able to both store and manipulate the orbital angular momentum (OAM) of an input field, obtaining outputs with topological charges that were algebraic functions of the input topological charges. For example, one can retrieve the double or triple of the input topological charges for the memories based on the χ(⁵) or χ(⁷) processes, respectively. The mechanisms of the nonlinear atomic memory are theoretically analyzed since the creation of the grating until the extraction of the field D, permitting a quantitative calculation of the im- portant physical features. Since such higher-order nonlinearities are associated with processes involving a larger number of photons, this kind of memory can in principle be used to generate multiple quantum-correlated photons, opening the possibility to extend the capability of many quantum information protocols. In this context, we propose a concrete method to gen- erate triplets of correlated photons based on a χ(⁵) memory. Finally, we also demonstrate that a Laguerre-Gauss mode can be amplified through a nonlinear stimulated Raman process. This could be important for applications in classical communication using OAM.